This response of fH to decreasing dissolved oxygen levels has been studied in other salmonids at similar temperatures (10–12°C; e.g. Oncorhynchus mykiss; Holeton and Randall, 1967; Marvin and Heath, 1968; Randall and Smith, 1967), and the onset of bradycardia for this species (∼50–65% air saturation) is very similar to what we report for Atlantic salmon in respirometers post-surgery and when free-swimming. This evidence concerns the gene FH and Bradycardia.